The present invention relates to a chain guide for a motor driven chain saw, the guide being in the form of an elongated rail defining a rear portion, an intermediate portion and a front portion, a longitudinal center axis and a peripheral surface for slidably supporting a saw chain, the rear portion being provided with attachment elements engageable with a holding fixture in a housing of the chain saw, the peripheral guiding surface defining a forward-run section extending in an upper part of the intermediate portion substantially parallel to the center axis and terminating at an end point, a return-run or cutting section extending from a start point in a lower part of the intermediate section substantially parallel to the center axis, and an arcuate tip section which includes a circular run-in section starting at the end point and terminating at a vertex point above the center axis, and a curved run-out section starting at the vertex point and terminating at the start point of the cutting section.
Recently it has been for the first time achieved to design a chain guide which is symmetrical relative to its center axis and which is shaped in such a manner that only by its geometric configuration the dangerous tendency to kick back inherent in all prior art power chain saws has been completely eliminated (DE-DM 88 03 810). This surprising advantage has been achieved by means of dimensioning the radius of curvature of the tip of the rail in the range of its vertex to amount 10 to 30 mm and to connect in this range with an end point of the forward-run section by a transmission section whose radius of curvature is at least 150 mm and which forms with the center axis of the rail an angle between 10.degree. to 40.degree.. However, this advantage of the complete elimination of the backstroke or kickback is accompanied with a significant disadvantage for practical operation, namely that during plunge-cutting operation the tip of the guide rail can be plunged into the wood only at the cost of a high power consumption.
From prior art (U.S. Pat. No. 3,323,561, DE-AS 15 03 968) guide rails of the above described type are known which in general have an asymmetric configuration relative to their center axes and are better suitable for the plunge-cutting operation. In particular, they have succeeded to eliminate vibrations or shaking motions of conventional chain saws which when using a symmetrical chain guide may lead even to the dislodging of the chain from the processed cut. In prior art asymmetric guide rails, however, there has been no possibility to eliminate kickbacks because the transfer of features of the known configuration of the guide rail design for avoiding the back stroke to the asymmetric guide rails would neutralize the achieved advantages of the latter as regards the plunge-cutting operation.